1. Field of the Invention
The present invention relates to control of a machine tool, and more particularly to device and method for controlling a feed control axis and a rotary control axis, both provided in a machine tool in which the rotary control axis is fed along the feed control axis.
2. Description of the Related Art
A machine tool provided with a feed control axis performing a linear or rotational feed motion and a rotary control axis adapted to be fed along the feed control axis and performing a rotary indexing motion has been known. The feed control axis carries a structural member such as a linear slide or a rotational slide, and the rotary control axis carries a structural member such as a rotary indexing table or a turret tool rest.
In the above machine tool, the feed control axis and the rotary control axis may exert a mechanical or dynamic effect (i.e., an interference) resulted from their respective motions on each other, which may affect the stability of position control of each axis and thus may result in the deterioration of the processing accuracy of a workpiece. Conventionally, in a technical field other than a machine tool, several measures for eliminating the interference between control axes structurally correlated with each other have been proposed.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2000-243811 (JP-A-2000-243811) discloses a stage device incorporated into an exposure apparatus, which is configured to mechanically prevent vibration in an optical system caused due to a reaction force exerted by a slider on a stator, both provided in a linear motor for driving a stage. The stage device includes a reaction device that generates, in the stator, a force canceling the reaction force from the slider. The reaction device controls a motion of the stator based on a detection value of a position detector for detecting an actual displacement of the stator, so as to appropriately eliminate not only the reaction force, but also other error factors and thus to stably keep the stator located at a predetermined position.
Japanese Unexamined Patent Publication (Kokai) No. 63-314606 (JP-A-63-314606) discloses a controller provided in an articulated robot, which can eliminate interference between joints (i.e., control axes) by a feedback compensation. The controller calculates an interference torque value generating in each control axis and estimates, by state observation means, nonlinear disturbance torque applied to each control axis as a correction value, and thereby corrects a torque command by using the interference torque value and the correction value.
Japanese Unexamined Patent Publication (Kokai) No. 09-222910 (JP-A-09-222910) discloses a controller provided in a multi-axis industrial robot in which each control axis is equipped with a spring element such as a decelerator. The controller estimates, by state observation means disposed in each axis, a twist angle between an electric motor and a load, calculates an interference force by using the estimated twist angle, and corrects a torque command given to the electric motor based on the interference force.
In the stage device disclosed in JP-A-2000 -243811, the stator, that should naturally be immovably supported, is intentionally mounted on a motion control axis (i.e., the reaction device) for the purpose of canceling the reaction force exerted by the slider on the stator in the linear motor. Therefore, the relationship between a linear feed-control axis of the slider and the motion control axis of the stator is somewhat different from the relationship between the feed control axis and the rotary control axis in the above-described machine tool to which the invention of the present application belongs. Also, in the stage device, the position detector is used to measure the actual displacement of the stator for the purpose of eliminating various error factors in the motion control of the stator. However, in the case where such a position detector is installed in a machine tool, equipment costs of the machine tool may increase, and since the machine tool is often placed in poor environments, problems in ensuring installation space or maintaining reliability of the position detector may arise. Moreover, it is difficult to adapt a technique for correcting the command value given to the stator, on the basis of the actual value of the displacement of the stator, to a quick and accurate positioning control for a movable section in a typical machine tool.
Also, in the robot controllers disclosed in JP-A-63-314606 and JP-A-09-222910, data representing a state in which the electric motor actually operates (i.e., a state quantity) are used as several data required for calculating the interference force or correcting the command value. Such a configuration may be effective for controlling a robot, in which the moving speed or positional accuracy of an arm is lower than that of the movable section in the typical machine tools, but may also be difficult to be adapted to the control of the machine tool requiring the quick and accurate positioning.